This invention relates in general to electrically-heated catalyst structures and methods for starting up a gas turbine engine. Catalytic combustion systems have been proposed for turbines or engines which burn hydrocarbonaceous fuels. The catalytic combustion systems are used to reduce the amount of uncombusted hydrocarbons and other undesirable combustion by-products such as carbon monoxide and nitrous oxides (NOx). Typically, catalytic combustion systems are quite effective in reducing the amount of these undesirable emissions once steady-state operating conditions are achieved. However, during the initial start-up of the turbine or engine, the amount of emissions may be above the desired limits.
The amounts of undesirable emissions are usually higher during start-up conditions because the catalyst is not at a temperature at which it is most effective. One method which has been used to achieve lower emissions is to more quickly bring the catalyst up to its desired operating temperature by preheating the air that is supplied with the hydrocarbonaceous fuel to the catalytic combustion system. Another way to achieve lower emissions more quickly is to preheat the catalyst.
Typically, a catalytic structure is a metallic or ceramic substrate, coated with a catalytically reactive substance and placed into a fuel/air stream. The catalysts are typically Group VIII noble metals or the platinum group metals and react with the fuel/air mixture passed over the catalyst structure. The reaction rate of the fuel/air mixture over the catalyst is temperature dependent, typically being low or non-existent at low temperatures, most efficient and controllable in a particular higher temperature range, while above certain known high temperatures the catalyst suffers deactivation and/or the reaction becomes uncontrollable. Therefore, it is important to design the catalytic structure so that it will withstand the operating temperatures of the system it is installed in, can be quickly brought up to its most efficient operating temperature range and yet, be maintained within this desired operating temperature range so as to prevent any catalytic deactivation and/or runaway reaction.
Until the catalyst reaches an ideal operating temperature range, the combustion of the fuel in the fuel air mixture is likely to be incomplete. It has been shown that by heating the catalyst structure prior to or concurrently with the introduction of the fuel/air mixture, the catalyst can be brought up to a satisfactory operating temperature range, with a concurrent increase in the amount of fuel combusted. This, in turn, leads to a reduction in the amount of emissions during the start-up period. It is known in the art to electrically heat a catalytic structure to reduce emissions during start up of a gas turbine. For example, U.S. Pat. No. 5,440,872 to Pfefferle describes a catalyst designed to lower the start up emissions of a gas turbine using a catalytic combustion system. Pfefferle '872 uses a "microlithic catalyst" with a very low mass so that heat-up occurs quickly. Pfefferle also proposes the use of electrical heating to raise the catalyst temperature prior to introducing fuel. Because the catalyst operates at the adiabatic combustion temperature of the fuel/air mixture, the system is limited to temperatures in the range of 600.degree. K. to 1250.degree. K. (327.degree. C. to 977.degree. C.). This is a low temperature limiting this technology to gas turbines with low turbine inlet temperatures. The technology of the present invention is not limited to low combustor outlet temperatures and has been demonstrated at combustion outlet temperatures as high as 1500.degree. C.
A further example of an electrically-heated catalyst is disclosed in U.S. Pat. No. 5,070,694 to Whittenberger. Whittenberger '694 describes an electrically-heated catalytic structure comprised of alternating strips of brazing material and thin metal strips, all of which are fused to a central electrode. The foil unit is made catalytically reactive by dipping it in a bath which contains slurries of the catalytic coating, and then it is spirally wound and encased in an electrically conductive outer shell. Current is then passed from the outer shell to the center electrode to heat the structure. This device is designed for use in an automotive application and uses a voltage source in the range of 12 to 24 volts and a start-up temperature of approximately 650.degree. C. The structure of the catalyst in whittenberger '694, in which all of the surfaces are catalytically reactive and are not insulated from one another, proscribes its use in an application using high voltages since short circuiting and uneven heating may occur in this structure. In contrast, the structure of the present invention provides insulative barriers between the current carrying members to ensure there is no short circuiting. In addition, the structure of the present invention can be operated at voltages of about 100 volts or higher with the structure being evenly heated with no damage from arcing or overheating.
The above examples of electrically-heated catalyst units have used a centrally located electrode to complete the electrical flow path. It is known that in applications using catalytic structures in gas turbines, any irregularity in the structure of the catalyst bed such as an electrode, can cause flow disruptions or irregular flow patterns leading to hot spots or even premature ignition of the fuel/air mixture and thereby destroying the structure. An electrically-heated catalytic structure that does not use a central electrode is described in U.S. Pat. No. 5,232,671 to Brunson et. al. Brunson '671 describes a spirally wound structure having two groups of catalytically reactive foils separated by an insulating barrier. Each of the foils in the separate groups is connected to one pole of the voltage source, and all the foils are connected to each other in the center of the structure. This method of connection puts each foil in parallel to the other foils, minimizing the available resistance of the foils. The foils in Brunson '671 are also connected in the center of the structure by a pin or other type of crimping device. In contrast, the technology of the present invention has the foils arranged in series with each other to maximize the available resistance. An embodiment of the present invention also provides a structure without a central supporting member such as an electrode or a pin.
U.S. Pat. No. 5,250,489 to Dalla Betta et. al. discloses a catalytic structure that is formed into a spiral shape and which has integral heat exchange. In contrast to the present invention, this disclosure is not an electrically-heated catalyst (EHC), and the disclosure does not teach or suggest any requirements regarding the electrical considerations and insulative properties required in an EHC.